The present invention relates to optical waveguide sensors. In particular, it relates to optical waveguide sensors having first and second cladding regions, each cladding region having an effective refractive index such that the total effective refractive index of the two cladding regions is approximately equal to but not greater than the environmental refractive index to be sensed.
Optical waveguide sensors are finding increased applications in civil, industrial, and military fields where enhanced sensitivity, geometrical flexibility, miniaturization, immunity from electromagnetic interference and multiplexing capabilities are desirable. Some of these sensors are fabricated by exposing an optical waveguide to a pattern of light at a wavelength at which the waveguide is photosensitive. The light pattern photoinduces a refractive index variation or grating with the same spatial profile as the light pattern. Long period gratings have periods (10-1500 microns) that are greater than the operating light source wavelength. In use, long period gratings convert light traveling in the forward-propagating guided mode of the waveguide to forward-propagating, non-guided lossy cladding modes of the waveguide.
Vengsarkar (U.S. Pat. No. 5,430,817) first proposed the use of long period gratings for optical fiber communication systems and in optical fiber sensing systems. The sensing system comprised a source of optical energy and a length of optical fiber that included both a short period reflective sensing grating for reflecting light and a long period grating coupled to the fiber for receiving light reflected from the short period grating. A photodetector was used for detecting the intensity of the light through the device.
Later, Vengsarkar et al. (U.S. Pat. No. 5,641,956) proposed an optical waveguide sensor arrangement that did not require the use of a short period or Bragg grating. This arrangement comprised an optical waveguide sensor having guided modes, lossy non-guided modes, and a long period grating coupling the guided modes to the lossy non-guided modes. The long period grating produces a wavelength transmission spectrum functionally dependent on the physical parameter sensed. In turn, this optical sensor arrangement is useful for measuring temperature, strain, shape, refractive index and corrosion. However, this type of arrangement has its limitations in that the maximum refractive index sensitivity can only be obtained in the refractive index range approximately equal to that of the fiber cladding glass used or an index near and below 1.44. When highly sensitive measurements are necessary, such as measurements with respect to biological samples, which have a refractive index approximately equal to 1.33, these sensors fail to produce a notable change in the coupled wavelength because the refractive index of 1.33 is not approximately equal to and below 1.44, for example 1.43. Rather, in order to use this type sensor, one would have to alter the environmental refractive index by seeding the environment with titanium or some other high refractive index particles. Thus, the need exists to have an optical sensor which has a high refractive index sensitivity and operates in a refractive index range from about 1.1 to about 3.0.
DiGiovanni et al. (U.S. Pat. No. 5,802,236) describe a microstructured optical fiber that has a solid silica core region surrounded by an inner cladding region and an outer cladding region. The inner cladding region has larger ratio of void to glass than the outer cladding region. Consequently, the inner cladding region has a lower effective refractive index than the outer cladding region, and both cladding regions have a lower effective refractive index than the core region. The effective refractive index of a feature of a fiber is defined as the value of the refractive index of the feature that gives, in a simulation of the fiber, the same optical properties as the actual fiber. Roughly speaking, the effective refractive index of a non-homogeneous material can be considered to be a weighted average of the refractive indices of the constituents of the material. However, when considering the total effective refractive index of the cladding of an optical fiber, one must sum each individual effective refractive index for each cladding. In so doing, the total effective refractive index of the microstructured fiber disclosed by DiGiovanni et al. is driven by light propagation parameters and is arbitrary with respect to the environment.
Some have tried to fulfill the need for high refractive index sensitivity at lower ambient refractive indices by coupling light into higher order modes. These modes have a larger evanescent field outside the fiber so changes in the external refractive index have a greater influence on the effective index of the coupled cladding mode. As a result, a larger wavelength shift has been obtained with higher-order modes when compared to lower order modes. However, coupling into higher-order modes has limitations. First, coupling into higher order modes depends on the modal overlap between the fundamental mode and the cladding mode. Each mode has an energy distribution and for modal coupling to occur, there must be an overlap of this energy distribution. For higher order modes, this overlap with the fundamental mode is reduced. Therefore, to couple light into these modes, better coupling conditions are required which are sometimes difficult to obtain. Furthermore, coupling into higher-order modes does not necessarily increase sensitivity beyond certain higher order modes.
One method to increase long period grating sensitivity is to couple light into a cladding mode near where the group index of the core equals the group index of the cladding. At this point, the slope of the long period grating characteristic curve is infinite which corresponds to maximum long period grating sensitivity with respect to the refractive index. However, operating at or near this point causes a broadening of the long period grating spectral peak which reduces the sensitivity one is trying to obtain. The response of the long period grating is also non-linear and is dependent upon the effective refractive index of the cladding. Moreover, this non-linear response is asymptotic at the effective refractive index of the cladding. Since fused silica glass is often used in fiber, the refractive index is near 1.44 and thus high sensitivity can only be obtained when sensing refractive index changes around 1.44.
Lastly, many sensors employing long period gratings use standard step index fibers or other radially symmetric fibers. Thus, the coupling from the fundamental mode to cladding modes can be determined through finite element method (FEM) models or by other means of solving the wave equation. Therefore, it is easy to determine long period grating characteristics and how the environment acts to change long period grating coupling conditions. When there is no longer a radially symmetric cladding, perturbations in the cladding make it difficult to predict how the evanescent field will interact with the environment.
An object of the present invention is to provide an optical waveguide sensor having an inner and an outer cladding region, each cladding region having an effective refractive index such that the total effective refractive index of the combined cladding regions is approximately equal to but not greater than the effective refractive index of a particular environment to be sensed.
Another object of the present invention is to provide an optical waveguide sensor having a total effective refractive index of the cladding regions ranging from about 1.1 to about 3.0.
Another object of the present invention is to provide an optical waveguide sensor which is prepared from a microstructured optical fiber that has a total effective refractive index approximately equal to but not greater than the effective refractive index of a particular environment to be sensed.
Another object of the present invention is to provide a process for measuring changes in an environmental parameter which employs an optical waveguide sensor having first and second cladding regions such that the total effective refractive index of the cladding regions is approximately equal to but not greater than the effective refractive index of the environmental parameter to be sensed.
By the present invention, an optical waveguide sensor is presented that exhibits a high sensitivity over a range of ambient refractive indices. This waveguide is useful for highly sensitive applications such as the testing of biological materials. In its simplest configuration, the optical waveguide has a core having at least one long period grating disposed therein. Each long period grating has a plurality of index perturbations spaced apart by a periodic distance xcex9, wherein the periodic distance is 10 xcexcmxe2x89xa6xcex9xe2x89xa61500 xcexcm. An inner cladding region surrounds the core. The inner cladding region has a plurality of spaced apart first cladding features. These features are disposed within a first cladding material. The resulting inner cladding region has an effective refractive index. The waveguide also comprises an outer cladding region which surrounds the inner cladding region. The outer cladding region has a plurality of spaced apart second cladding features. These second cladding features are disposed within a second cladding material. The resulting outer cladding region has an effective refractive index. The total effective refractive index of the inner cladding region and the outer cladding region is approximately equal to but not greater than the effective refractive index of an environmental parameter to be sensed.
The optical waveguide sensor is coupled to a source means for providing light to the optical waveguide sensor. An optoelectronic detector is positioned in an operable relationship to the optical waveguide sensor and detects light transmitted through the optical waveguide sensor.
In use, this sensor is employed to measure changes in an environmental parameter. This is accomplished by providing the optical waveguide sensor, exposing the sensor to an environmental parameter, and measuring a change in the long period grating coupling conditions.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.